This invention relates generally to a piezoelectric composite material and, more specifically, to a flexible piezoelectric composite material containing a blend of piezoelectric ceramic powder homogeneously dispersed in an organic matrix and suitable for use in underwater acoustic transducers adapted to generate acoustic waves or supersonic waves or to receive such waves propagating in water.
Piezoelectric composite material containing piezoelectric ceramic powder, such as PbTiO.sub.3 or Pb(Ti,Zr)O.sub.3 powder, with an average particle size of several .mu.m homogeneously dispersed in an organic matrix such as a synthetic rubber is known. This composite material has a high piezoelectric constant and a good flexibility and shows good acoustic compatibility with water because of its low density and, hence, is suitable for utilization for underwater acoustic transducers.
The piezoelectric characteristics of such a piezoelectric composite material can be enhanced as the content of the piezoelectric ceramic powder in the matrix is increased. However, when the piezoelectric ceramic powder is used in a large amount, the amount of pores contained in the composite material is increased to the extent that the piezoelectric characteristics vary according to the hydrostatic pressure to which the composite material is subjected during use. To cope with this problem the present inventors have proposed the use of two kinds of ceramic particles with different average particle sizes (Japanese Patent Applications Nos. Sho 63-223342 and Hei 1-288442). Since small particles can fill the space between large particles, the amount of pores can be decreased so that the resulting composite exhibits less pressure dependency. However, the reduction of the pressure dependency and the piezoelectric characteristics of this composite material are still not fully satisfactory.
In hydrophones using a piezoelectric composite material of the above-mentioned type, the sensitivity is expressed as d.sub.h (=d.sub.33 +2d.sub.31). A piezoelectric composite material showing a large d.sub.h value has a high sensitivity. For the purpose of increasing the d.sub.h value, there have been proposed a method in which a piezoelectric ceramic having a large piezoelectric constant, such as Pb(Ti,Zr)O.sub.3, is used, a method in which the amount of the piezoelectric ceramic is increased and a method in which the polarization voltage is increased. Up to present, however, piezoelectric composite material having sufficiently high sensitivity and low pressure dependency has not been provided yet.
In known piezoelectric composite materials, when the strain in one direction, for example, d.sub.33 direction (in the direction parallel with the electrical field, is increased, the strain in the other direction, for example, d.sub.31 direction (in the direction perpendicular to the electrical field) is also increased as a result of lateral-longitudinal coupling. Thus, in the case of a hydrophone which is to be connected to a hull and displaced therewith, noises attributed to the strain in the d.sub.31 direction are unavoidably generated.
More particularly, a piezoelectric composite sheet obtained by impressing a direct current voltage of 70 kV/cm between opposing electrodes having disposed therebetween a sheet-like piezoelectric composite containing 60% by volume of Pb(Ti,Zr)O.sub.3 powder dispersed in an epoxy resin matrix has d.sub.33 and d.sub.31 of 104.times.10.sup.-12 C/N and -45.5.times.10.sup.-12 C/N, respectively. When such a composite sheet is used in air, the wave receiving sensitivity thereof is generally equal to d.sub.33. On the other hand, when used in water, the composite sheet is subjected to hydrostatic pressure in all directions so that the sensitivity is d.sub.h which is d.sub.33 +2d.sub.31 as described above, namely 13.times.10.sup.-12 C/N. Thus, the sensitivity is reduced to about 1/8 of d.sub.33. Furthermore, when such a composite is used as a hydrophone, a noise signal is generated due to the strain in d.sub.31 direction. This noise is added to the positive signal generated by the strain in the d.sub.33 direction to lower the Signal-to-Noise ratio.